1. Technical Field of the Invention
The present invention relates to a plasma light source for EUV (Extreme Ultra Violet) radiation and a plasma light generation method.
2. Description of the Related Art
Lithography using EUV light sources has been expected for microprocessing of next-generation semiconductors. Lithography is a technique to fabricate an electronic circuit by reduction-projecting light or beam onto a silicon-substrate through a mask with a circuit pattern drawn thereon to expose a resist material to light. Minimum processing dimensions of a circuit fabricated by optical lithography basically depend on the wavelength of a light source. Thus, a shorter-wavelength light source is essential for the development of next-generation semiconductors, and research has been carried out for the development of such light sources.
A highest potential light source for next-generation lithography is an EUV light source, meaning light in the wavelength ranging from about 1 to 100 nm. Since light in this range has a high absorptance for all substances so that transmission optical systems such as a lens cannot be used, a reflection optical system has to be used. It is very difficult to develop optical systems in an EUV range, and such optical systems show a reflection property only for limited wavelengths.
A reflecting mirror formed by multilayer of Mo/Si to have sensitivity at 13.5 nm has been already developed, and it is expected that the development of a lithography technique using light of this wavelength in combination with the reflecting mirror will enable processing dimensions of 30 nm or less. To realize further advanced microprocessing techniques, the development of a lithography light source with the wavelength of 13.5 nm is urgently required, and radiant light from high energy density plasma attracts attention.
Generation methods for light source plasma can be classified roughly into laser produced plasma (LPP) types and discharge produced plasma (DPP) types driven by a pulsed power technique. DPP has an advantage of a conversion efficiency superior to LPP because input electric power is directly converted into plasma energy, and has an advantage of achieving a compact device and low cost.
A conversion efficiency (Plasma Conversion Efficiency: P.C.E) from plasma to radiant light in an effective wavelength band (in-band) is represented by the following expression (1):P.C.E=(Pinband×τ)/E  (1),
where Pinband is an EUV radiant light output in the in-band, τ is radiation duration, and E is energy input to plasma.
Typical elements having radiation spectra in the in-band include Xe, Sn, Li and the like. At an early stage of the development, research on Xe was mainly conducted because of the ease of handling. In recent years, however, Sn has attracted attention because of high output and high efficiency, and research on Sn is now being conducted. Additionally, there have been growing expectations for hydrogenlike Li ions (Li2+) having a Lyman-α resonance line just in the in-band area.
Radiation spectra from high-temperature and high-density plasma are basically decided by a temperature and a density of a target substance. According to a result of the calculation on atomic process of plasma, for generating plasma in the EUV radiation region, optimum electronic temperatures and electronic densities are 10 to 30 eV and 1018 cm−3, respectively, for Xe and Sn, and 20 eV and 1018 cm−3, respectively, for Li.
The above-mentioned plasma light sources are disclosed in Non-Patent Documents 1 and 2, and Patent Document 1.
[Non-Patent Document 1]
SATO Hiroto et al. “Discharge-Produced Plasma EUV Source for Lithography”, OQD-08-28
[Non-Patent Document 2]
Jeroen Jonkers, “High power extreme ultra-violet (EUV) light sources for future lithography”, Plasma Sources Science and Technology, 15(2006) S8-S16
[Patent Document 1]
Japanese Patent Publication No. 2004-226244, “Extreme ultra-violet light source and semiconductor aligner”
EUV lithography light sources are required to have a high average output, a small size of light source, less flying particles (debris), and so on. In the current state, a generated EUV light amount is extremely lower than a required output, and so one of big challenges is to develop light sources with higher output. On the other hand, an increase in input energy for higher output will cause degradation in lifetime of a plasma generator and an optical system due to damage by heat load. Therefore, in order to realize both of a high EUV output and low heat load, high energy conversion efficiency is essential.
At an early stage of plasma formation, a lot of energy is consumed by heating and ionization. Additionally, since typical plasma in a high-temperature and high-density state for EUV radiation expands rapidly, the radiation duration τ is extremely short. In order to improve the conversion efficiency, it becomes important to sustain plasma for a long time (in the order of microseconds) in a high-temperature and high-density state suitable for EUV radiation.
Media such as Sn and Li that are solids at room temperatures have a high spectrum conversion efficiency. On the other hand, since such media cause a phase change such as melting and evaporation in plasma formation, influences of debris such as neutral particles (derivatives from discharging) on contamination in the device are increased. Therefore, improved target supply and recovery system also is required.
Currently a typical radiant period of an EUV plasma light source is about 100 nsec, and the output is extremely less than requirement. In order to satisfy both of high conversion efficiency and high average output for industrial application, an EUV radiant period of 1 to 5 μsec has to be achieved with one shot. That is to say, in order to develop a plasma light source with high conversion efficiency, plasma in a temperature and density state suitable for each target has to be constrained for 1 to 5 μsec (at least 1 μsec), and stable EUV radiation has to be achieved.
Further, conventional capillary discharge has a drawback of the small effective radiant solid angle because plasma is confined in a capillary.